Binoculars, as an example of optical instruments, are classified into binoculars with fixed magnification and binoculars with variable magnification or so-called zoom binoculars.
Zoom binoculars are equipped with many lenses, compared with fixed magnification scopes, so as to have good optical performance irrespective of their varied magnifications, as disclosed in Patent Document 1. Also, zoom binoculars are provided with complicated mechanisms for moving lenses to vary the magnification without changing their focal point.    Patent Document 1: JP H10-246858 A